1. Field of the Invention
The invention relates to the field of Radio Frequency Transponders and more specifically to retaining and restoring of valid state information by the Radio Frequency Transponders upon the reapplication of power.
2. Description of the Background of the Invention
Radio Frequency (RF) Transponders (tags) are used in a multiplicity of ways. They may be used in locating and identifying accompanying objects, as well as for transmitting information about the state of an object. It has been known since the early 60""s that electronic components of transponders could be powered by a sequence of periodic signal bursts sent by a xe2x80x9cbase stationxe2x80x9d and received by a tag antenna on each of the transponders.
The RF electromagnetic field induces an alternating current in the transponder antenna that can be rectified by a RF diode of the transponder, and the rectified current can be used for a power supply for the electronic components of the transponder. The current induced in the transponder antenna from the incoming RF energy would thus be changed, and the change in the alternating current changes or modulates the RF power radiated from the transponder antenna back to the base station. This change in the radiated power from the transponder antenna is picked up by the base station antenna. Thus, the transponder antenna broadcasts a return signal without itself having a self contained power supply.
The xe2x80x9crebroadcastxe2x80x9d of the incoming RF energy is conventionally called xe2x80x9cback scatteringxe2x80x9d, even though the transponder broadcasts the energy in a pattern determined solely by the transponder antenna. Since this type of transponder carries no power supply of its own, it is called a xe2x80x9cpassivexe2x80x9d transponder to distinguish it from a transponder containing a battery or other energy supply, conventionally called an active transponder.
As an example, consider an RF tag designed to respond equally well to signals at frequencies ranging from 2.425 GHz to 2.475 GHz in an ideal environment. In a less then ideal environment, where there are other RF tags, metal objects, water-filled objects, etc. disposed in close proximity to a particular RF tag that is attempting to communicate with the base station, the RF energy available at a particular frequency to be received by the particular RF tag may be attenuated. This situation is analogous to repositioning a TV or a radio antenna to get the strongest reception. Similar to the effect of the person repositioning the antenna on the reception of the antenna, the presence of other tags and/or objects interferes with the RF reception of a particular tag. Additionally to comply with the FCC regulations, the carrier frequency used by the base station hops over relatively narrow channels of up to 1 MHz wide in the allowed band, e.g., 2.400 to 2.483 GHz in the 2.450 GHz case, during communication.
When an array of tags is being interrogated by a base station, it is possible for very different field strengths to be available to tags depending on the carrier frequency of that channel being used by the base station at the time of the communication and on the different positions of tags in the array. For instance, a first tag at a first position may be well powered when the base station operates at 2.422 and 2.463 GHz but not at 2.447 GHz, while a second tag at a second different position may be well-powered when the base station operates at 2.463 GHz and 2.447 GHz, but not 2.422 GHz. These differences are related not to the RF tag design but to the instantaneous RF environment of the individual RF tag at the time the interrogation by the base station occurs.
If power being supplied to the RF tag has been removed for even short time duration, the state information being maintained or stored by the RF tag is lost. For example, when the RF burst powering the RF tag falls off, the tag power, which for passive tags is maintained by a storage capacitor, may be lost in as little as 100 microseconds. The state information of the RF tag is then also lost.
Losing the state information of the RF tag is particularly injurious when a base station sending a polarized RF is interrogating an array of RF tags having antennas polarized in different manners. When some RF tags may not be powered up by a particular frequency used, the communication protocol will attempt to talk to each tag in the array.
RF tags may have major and minor states. The major states may include the xe2x80x9cID,xe2x80x9d xe2x80x9cREADY,xe2x80x9d and xe2x80x9cDATA-EXCHANGExe2x80x9d states. Each RF tag identifies itself to the base station in the xe2x80x9cIDxe2x80x9d state, lets the base station know that it is ready to transfer data in the xe2x80x9cREADYxe2x80x9d state, and sends data in the xe2x80x9cDATA-EXCHANGExe2x80x9d state. The minor states include information such as the counter value used during the identification protocol initiated from the base station.
When a RF signal burst of a first frequency is sent from the base station to an array of RF tags, some of RF tags in the array do receive sufficient power to operate from that signal burst and will proceed to operate through the stages or states of operation, such as entering the xe2x80x9cREADYxe2x80x9d or xe2x80x9cDATA-EXCHANGExe2x80x9d states. The RF tag is typically operated cyclically through those states; in each cycle the states are carried out in the order set out above. Thus if the base station knows in which state a particular tag is operating, it has an effective xe2x80x9cbook arkxe2x80x9d as to where in the cycle this particular tag is operating. When the RF environment changes or when the base station hops to new carrier frequency, some of the tags that were previously powered, will not now receive sufficient power and will no longer be able to operate. At the same time, other RF tags in the array that previously had insufficient power to operate will now become powered up by the RF burst of the new frequency and start working.
An illustrative cycle of operation of the array of ten RF tags may be described as follows:
1. The base station or the reader is on channel one and RF tags 1-8 respond by beginning their participation in the identification protocol. All eight tags are successfully identified.
2. The reader now hops to channel two, the frequency of channel 2 allows tags 7-9 to be powered. Tag 9 will now respond by beginning participation in the identification protocol, while tags 1-6 lose their power and therefore stop participating. Since tags 7 and 8 were already identified and continue to be powered sufficiently when operating on channel, they do not participate in the protocol.
3. The reader hops to channel 3. The frequency of channel 3 allows tags 2-10 to be powered. Tags 7-9 stay powered and do not participate in the protocol. However, tags 2-6 must be reidentified in order to identify the one truly new tag 10.
The RF tags that are not well powered lose track of state information. This state information is essentially a bookmark in the communication sequence between the tag and the base station. In running an ID protocol, for example, tags that newly enter the field as well as tags that have lost power and then regained it while remaining in the field are treated equally; they both have to be identified from scratch, wasting time. If state information could be maintained, the tags that remain in the field and are not powered sufficiently even only for brief periods of time would not have to reenter the protocol and thus system level performance with regard to tag identification would be improved.
What is needed is for the state information of each RF tag to be maintained in order to prevent that RF tag from participating in the protocol with the base station, in identifying previously unidentified tags of a tag array therefore improving system performance.
The present invention assures the integrity of state information retained by the RF tags during a loss of power. At the time of the regular operation of each RF tag, the power is provided to a voltage-storing device, such as a capacitor, powering an information retention mechanism of the RF tag. The power source for that voltage storing device may be directed through a device such as a diode or a PFET, which may be dedicated or shared with other components of the RF tag.
After the loss and reestablishing of power to the transponder but before the transponder restarts, the transponder checks the voltage-storing device powering the information retention mechanism to determine whether sufficient power is present to retain information without corruption. Additionally, after the power is restored, the delaying circuitry of the transponder may delay the restart to assure proper determination of whether sufficient power is present. If sufficient power is present, the transponder communicates a signal to indicate that fact, the stored information is restored and the transponder restarts.